Extruding a coating of a polyolefin or blends of polyolefins onto a substrate, such as paper or aluminum foil, to form an extrusion coated substrate is well known in the art. Various blends of polyethylene and other polyolefins have been successfully developed and used as extrusion coating compositions in the prior art. However, a polyolefin such as crystalline polypropylene alone, regardless of its molecular weight, i.e. flow rate, is not a satisfactory extruding coating composition since it does not exhibit high speed coating ability over a wide range of coating weights. Therefore, many of the excellent physical properties of polypropylene cannot be utilized in extrusion coating applications.
In order to improve the extrusion coating properties of polypropylene, blends of polyethylene and polypropylene have been developed and used as an extrusion coating. Typically, the PP/PE blends disclosed in the prior art comprise a blend of a visbroken polypropylene polymer and an ethylene homopolymer or copolymer or a blend of polypropylene and a homopolymer or copolymer of ethylene which is visbroken with a peroxide compound in an extruder. The term "visbroken" as used in the prior art means heating a polymer or a mixture of polymers to effect degradation via thermal cracking to produce a product having segments of lower molecular weight, evidenced by a greater flow rate. Hence, visbroken polymeric blends do not contain long chain branching therein.
U.S. Pat. No. 3,607,987 to Walton et al. provides an improved coating composition which comprises a blend of a major amount of a thermally cracked, i.e. visbroken, copolymer of ethylene and propylene, the copolymer containing no more than about 5 weight percent ethylene based oF the total weight of the copolymer and having a melt flow after visbreaking in the range from about 40 to about 110, with a minor amount of a low density homopolymer of ethylene.
U.S. Pat. No. 4,375,531 to Ross relates to blends of polypropylene based polymeric materials which are visbroken in order to impart both high impact and high melt flow rate properties to the polymeric material. Specifically, the visbroken polymeric compositions comprise a first component which is selected from the group consisting of block copolymers of propylene and ethylene, reactor-made intimate mixtures of polypropylene and randomly oriented copolymers of propylene and ethylene, and a second component which is selected from the group consisting of low density polyethylene, ethylene-vinyl acetate copolymers, acrylate-modified polyethylene, high density polyethylene, ethylene-propylene rubber (EPR or EPDM), and blends thereof.
Another typical blend that is used in the prior art is disclosed in U.S. Pat. No. 4,508,872 to McCullough et al. More specifically, McCullough et al. relates to modified propylene polymer compositions that have improved flow and impact resistance characteristics. The modified propylene polymer compositions disclosed in this reference are obtained by visbreaking a blend of an impact modified propylene polymer, a high density ethylene homopolymer (HDPE) and a linear low density ethylene copolymer (LLDPE). In a preferred embodiment, the impact-modified propylene polymer is visbroken separately and then melt blended with a mixture of HDPE and LLDPE.
Japanese Kokai Patent Application No. Sho 61-23634 relates to improved polypropylene composites useful for extrusion coating applications. The composites disclosed in this reference are made by mixing 60-95 parts by weight of a heat-treated polypropylene polymer which is obtained by heat treating polypropylene resins in the presence of peroxide and 5-40 parts by weight of polyethylene that has a crystallinity of greater than 25% as determined by X-ray analysis. Alternately, the compositions can be prepared by heat treating, in the presence of peroxide, a resin mixture which comprises 60-95 parts by weight polypropylene and 5-40 parts by weight of the foregoing mentioned polyethylene. The polypropylene composites produced in the japanese patent are said to exhibit improved moldability. It is emphasized that the term "heating treating" is used in this reference to denote visbreaking.
U.S. Pat. No. 5,066,723 to Randall, Jr. et al. provides impact-modified polymers which comprise visbroken reactor blends of free radical decomposable olefin polymers and free radical crosslinkable thermoplastic olefin polymers. More specifically, the impact-modified polymers are obtained by visbreaking a reactor blend of polypropylene and thermoplastic polyethylene materials. Such impact-modified polymeric materials are said to exhibit unexpected compatibility characteristics, increased impact resistance at low temperatures, improved antiblush characteristics and improved flow characteristics.
U.S. Pat. No. 5,218,046 to Audureau et at. relates to thermoplastic compositions that exhibit improved mechanical properties such as low temperature resilience, elongation and tensile strength and a method for producing the same. Briefly, the process disclosed in Audureau et al. involves visbreaking an ethylene polymer or ethylene copolymer and then mixing the visbroken product with a propylene polymer or propylene copolymer.
Despite the current state of the art, none of the references noted hereinabove disclose a process for preparing high flow rate thermoplastic polymeric compositions, which exhibit superior melt strength and reduced draw resonance behavior, by melt mixing above the decomposition temperature of the peroxide, a mixture comprising a high propylene content polymeric component and a non-crosslinked polymeric component which has previously been intimately precontacted with a peroxide compound well below the decomposition temperature of the peroxide and the melting point of the polymers, as does the present invention.
Moreover, none of these references disclose the alternative method of the instant invention. That is, none of the references listed hereinabove disclose a process for preparing a peroxide PP/PE blend which comprises first blending the aforementioned high propylene content polymeric component and the non-crosslinked polymeric component and then intimately contacting the aforementioned blend with a sufficient amount of a peroxide to provide a peroxide adsorbed composition.
In contrast to the methods disclosed in the instant invention, all of the above-identified references require that the polymers or the polymer blends be visbroken. As mentioned hereinabove, polymers that are visbroken do not contain long-chain polymer branching. Consequently, the PP/PE blends described in the prior art do not exhibit the unexpected viscosity upturn in their dynamic melt rheological data at frequencies of less than about 1.0 rad/sec. which is exhibited by the blends of the instant invention.